Use sassc to compile CSS assets

Using sassc is the preferred method for new middleman projects https://github.com/middleman/middleman/issues/1903#issuecomment-216660657

Using middleman server --instrument yields huge differences.

For example, loading the biggest file style.css.scss before and after:

== Instrument (builder.output.resource.render-template): 2713.0550599999997ms {:path=>"styles.css.scss"}

== Instrument (builder.output.resource.render-template): 222.899554ms {:path=>"styles.css.scss"}